Serine Proteases
Serine proteases serve an important role in human physiology by mediating the activation of vital functions. In addition to their normal physiological function, serine proteases have been implicated in a number of pathological conditions in humans. Serine proteases are characterized by a catalytic triad consisting of aspartic acid, histidine and serine at the active site.
The naturally occurring serine protease inhibitors are usually, but not always, polypeptides and proteins which have been classified into families primarily on the basis of the disulfide bonding pattern and the sequence homology of the reactive site. Serine protease inhibitors, including the group known as serpins, have been found in microbes, in the tissues and fluids of plants, animals, insects and other organisms. Protease inhibitor activities were first discovered in human plasma by Fermi and Pemossi in 1894. At least nine separate, well-characterized proteins are now identified, which share the ability to inhibit the activity of various proteases. Several of the inhibitors have been grouped together, namely α1-antitrypsin-proteinase inhibitor, antithrombin III, antichymotrypsin, C1-inhibitor, and α2-antiplasmin, which are directed against various serine proteases, i.e., leukocyte elastase, thrombin, cathepsin G, chymotrypsin, plasminogen activators, and plasmin. These inhibitors are members of the α1-antitrypsin-proteinase inhibitor class. The protein α2-macroglobulin inhibits members of all four catalytic classes: serine, cysteine, aspartic, and metalloproteases. However, other types of protease inhibitors are class specific. For example, the α1-antitrypsin-proteinase inhibitor (also known as (α1-antitrypsin or AAT) and inter-α-trypsin inhibitor inhibit only serine proteases, α1-cysteine protease inhibitor inhibits cysteine proteases, and α1-anticollagenase inhibits collagenolytic enzymes of the metalloenzyme class.
Human neutrophil elastase (NE) is a proteolytic enzyme secreted by polymorphonuclear leukocytes in response to a variety of inflammatory stimuli. The degradative capacity of NE, under normal circumstances, is modulated by relatively high plasma concentrations of α1-antitrypsin. However, stimulated neutrophils produce a burst of active oxygen metabolites, some of which (hypochlorous acid for example) are capable of oxidizing a critical methionine residue in α1-antitrypsin. Oxidized α1-antitrypsin has been shown to have a limited potency as a NE inhibitor and it has been proposed that alteration of this protease/antiprotease balance permits NE to perform its degradative functions in localized and controlled environments.
α1-antitrypsin is a glycoprotein of MW 51,000 with 417 amino acids and 3 oligosaccharide side chains. Human α1-antitrypsin was named anti-trypsin because of its initially discovered ability to inactivate pancreatic trypsin. Human α1-antitrypsin is a single polypeptide chain with no internal disulfide bonds and only a single cysteine residue normally intermolecularly disulfide-linked to either cysteine or glutathione. The reactive site of α1-antitrypsin contains a methionine residue, which is labile to oxidation upon exposure to tobacco smoke or other oxidizing pollutants. Such oxidation reduces the biological activity of α1-antitrypsin; therefore substitution of another amino acid at that position, i.e. alanine, valine, glycine, phenylalanine, arginine or lysine, produces a form of α1-antitrypsin which is more stable. α1-antitrypsin can be represented by the following formula, SEQ ID NO. 63:
1         01         01         01         01        0MPSSVSWGIL LAGLCCLVPV SLAEDPQGDA AQKTDTSHHD QDHPTFNKIT PNLAEFAFSL YRQLAHQSNS TNIFFSPVSI ATAFANLSLG TKADTHDEIL100 EGLNFNLTEI PEAQIHEGFQ ELLRTLNQPD SQLQLTTGNG LFLSEGLKLV DKFLEDVKKL YHSEAFTVNF GDHEEAKKQI NDYVEKGTQG KIVDLVKELD200 RDTVFALVNY IFFKGKWERP FEVKDTEDED HVDQVTTVK  VPMMKRLGMF NIQHCKKLSS WVLLMKYLGN ATAIFFLPDE GKLQHLENEL THDIITKFLE300 NEDRRSASLH LPKLSITGTY DLKSVLGQLG ITKVFSNGAD LSGVTEEAPL KLSKAVHKAV LTIDEKGTEA AGAMFLEAIP MSIPPEVKFN KPFVFLMIEQ400 NTKSPLFMGK VVNPTQK417Ciliberto, et al. in Cell 1985, 41, 531-540. The critical amino acid sequence near the carboxyterminal end of α1-antitrypsin is shown in bold and underlined and is pertinent to this invention (details of the sequence can be found for example in U.S. Pat. No. 5,470,970 as incorporated by reference).
The normal plasma concentration of ATT ranges from 1.3 to 3.5 mg/ml although it can behave as an acute phase reactant and increases 3-4-fold during host response to inflammation and/or tissue injury such as with pregnancy, acute infection, and tumors. It easily diffuses into tissue spaces and forms a 1:1 complex with a target protease, principally neutrophil elastase. Other enzymes such as trypsin, chymotrypsin, cathepsin G, plasmin, thrombin, tissue kallikrein, and factor Xa can also serve as substrates. The enzyme/inhibitor complex is then removed from circulation by binding to serpin-enzyme complex (SEC) receptor and catabolized by the liver and spleen. Humans with circulating levels of α1-antitrypsin less than 15% of normal are susceptible to the development of lung disease, e.g., familial emphysema, at an early age. Familial emphysema is associated with low ratios of α1-antitrypsin to serine proteases, particularly elastase. Therefore, it appears that this inhibitor represents an important part of the defense mechanism against attack by serine proteases.
α1-antitrypsin is one of few naturally occurring mammalian serine protease inhibitors currently approved for the clinical therapy of protease imbalance. Therapeutic α1-antitrypsin has been commercially available since the mid 80s and is prepared by various purification methods (see for example Bollen et al., U.S. Pat. No. 4,629,567; Thompson et al., U.S. Pat. Nos. 4,760,130; 5,616,693; WO 98/56821). Prolastin is a trademark for a purified variant of α1-antitrypsin and is currently sold by Bayer Company (U.S. Pat. No. 5,610,285 Lebing et al., Mar. 11, 1997). Recombinant unmodified and mutant variants of α1-antitrypsin produced by genetic engineering methods are also known (U.S. Pat. No. 4,711,848); methods of use are also known, e.g., (α1-antitrypsin gene therapy/delivery (U.S. Pat. No. 5,399,346 to French Anderson et al.).
The two known cellular mechanisms of action of serine proteases are by direct degradative effects and by activation of G-protein-coupled proteinase-activated receptors (PARs). The PAR is activated by the binding of the protease followed by hydrolysis of specific peptide bonds, with the result that the new N-terminal sequences stimulate the receptor. The consequences of PAR activation depend on the PAR type that is stimulated and on the cell or tissue affected and may include activation of phospholipase C.beta., activation of protein kinase C and inhibition of adenylate kinase (Dery, O. and Bunnett, N. W. Biochem Soc Trans 1999, 27,246-254; Altieri, D. C. J. Leukoc Biol 1995, 58, 120-127; Dery, O. et al. Am J. Physiol 1998, 274, C1429-C1452).
TB and MAC
Mycobacterium is a genus of bacteria which are aerobic, mostly slow growing, slightly curved or straight rods, sometimes branching and filamentous, and distinguished by acid-fast staining. Typically, mycobacteria are gram-positive obligate aerobes. The genus mycobacterium includes the highly pathogenic organisms that cause tuberculosis (M. tuberculosis and sometimes M. bovis) and leprosy (M. leprae). There are, however, many other species of mycobacterium such as M. avium-intracellulare, M. chelonei (also known as borstelense and abscessus), M. africanum, M. marinium (also known as balnei and platypoecilus), M. buruli (also known as ulcerans), M. fortuitum (also known as giae, minetti, and ranae), M. haemophilum, M. intracellulare, M. kansasii (also known as luciflavum), M. littorale (also known as xenopi), M. malmoense, M. marianum (also known as scrofulaceum and paraffinicum), M. simiae, M. szulgai, and M. ulcerans. 
Mycobacteria which are pathogenic for animals but not believed to be pathogenic for humans include the following: M. avium-intracellulare (also known as brunense), M. flavascens, M. lepraemurium, M. microti, and M. paratuberculosis (which is the causative agent for Johne's Disease, and perhaps Crohn's disease). The following species of the genus mycobacterium are believed to be non-pathogenic: M. gordonae (also known as aquae), M. gastri, M. phlei (also known as moelleri and as timothy bacillus), M. nonchromogenicum, M. smegmatis, M. terrae, M. triviale, and M. vaccae. 
Additionally, certain mycobacteria other than M. tuberculosis and M. bovis are alternatively known as non-tuberculosis mycobacteria. They are divided into four groups, also known as Runyon groups, based on pigmentation and growth rate. Each group includes several species. Group I refers to slow-growing photochromogens; Group II refers to slow-growing scotochromogens; Group III refers to slow-growing nonphotochromogens; and Group IV refers to rapidly-growing mycobacteria. The non-tuberculosis mycobacteria are also called atypical or anonymous mycobacteria.
Tuberculosis is an acute or chronic infectious disease caused by infection with M. tuberculosis. Tuberculosis is a major disease in developing countries, as well as an increasing problem in developed areas of the world, with approximately 8 million new cases and 3 million deaths each year (See Styblo et al., Bull. Int. Union Tuberc. 56:118-125 (1981). Although the infection may be asymptomatic for a considerable period of time, the disease is most commonly manifested as an acute inflammation of the lungs, resulting in fever and a nonproductive cough. If left untreated, serious complications and death typically result.
Although it is known that tuberculosis can generally be controlled using extended antibiotic therapy, such treatment is not sufficient to prevent the spread of the disease. Infected individuals may be asymptomatic, but contagious, for some time. In addition, although compliance with the specific treatment regimen is critical, patient behavior is often difficult to monitor. Treatment regimens often require six to twelve months of uniterrurpted therapy. As a result, some patients do not complete the course of treatment, thus leading to ineffective treatment and development of antibiotic resistance. Effective vaccination and accurate, early diagnosis of the disease are needed in order to inhibit the spread of tuberculosis. Vaccination with live bacteria remains the most efficient method for inducing protective immunity. The most common Mycobacterium employed in the live vaccine is Bacillus Calmette-Guerin (BCG), an avirulent strain of Mycobacterium bovis. Some countries, such as the United States, however, do not vaccinate the general public because of concerns regarding the safety and efficacy of BCG.
M. tuberculosis is an intracellular pathogen that infects macrophages and is able to survive within the harsh environment of the phagolysosome in macrophages. Most inhaled bacilli are destroyed by activated alveolar macrophages. However, the surviving bacilli multiply in macrophages and are released upon cell death, which signals the infiltration of lymphocytes, monocytes and macrophages to the site. Antigenic stimulation of T cells requires presentation by MHC molecules. Lysis of the bacilli-laden macrophages is mediated by the delayed-type hypersensitivity (DTH) cell-mediated immune response and results in the development of a solid caseous tubercle surrounding the area of infected cells. Tuberculosis bacilli possess many potential T-cell antigens and several have now been identified [Andersen 1994, Dan. Med. Bull. 41, 205]. Some of these antigens are secreted by the bacteria. Continued DTH liquefies the tubercle, thereby releasing entrapped tuberculosis bacilli. The large dose of extracellular tuberculosis bacilli triggers further DTH, causing damage to the bronchi and dissemination by lymphatic, hematogenous and bronchial routes, and eventually allowing infectious bacilli to be spread by respiration.
Cell-mediated immunity to tuberculosis involves several types of immune effector cells. Activation of macrophages by cytokines, such as interferon-.gamma., represents an effective means of minimizing macrophage-based intracellular mycobacterial multiplication. However, this does not lead to complete eradication of the bacilli. Acquisition of protection against tuberculosis additionally requires T lymphocytes. Among these, T cells of both the CD8+ and CD4+ lineage appear to be particularly important [Orme et al, 1993, J. Infect. Dis. 167, 1481]. These T-cells secrete interferon-.gamma. in response to mycobacteria, indicative of a T.sub.h 1 immune response, and possess cytotoxic activity to mycobacteria-pulsed target cells. In recent studies using .beta.-2 microglobulin- and CD8-deficient mice, cytotoxic T lymphocyte (CTL) responses have been shown to be critical in providing protection against M. tuberculosis [Flynn et al, 1992, Proc. Natl. Acad. Sci. USA 89, 12013; Flynn et al, 1993, J. Exp. Med. 178, 2249; Cooper et al, 1993, J. Exp. Med. 178, 2243]. In contrast, B lymphocytes do not appear to be involved, and passive transfer of anti-mycobacterial antibodies does not provide any protective immunity. Thus, an effective vaccine regimen against tuberculosis must trigger cell-mediated immune responses.
Although commonly thought of only as a pulmonary infection, TB is well known to afflict many parts of the body. In addition to pulmonary TB, examples of other foci of tubercular infection include miliary TB (generalized hematogenous or lymphohematogenous TB), central nervous system TB, pleural TB, TB pericarditis, genitourinary TB, TB of the gastrointestinal tract, TB peritonitis, TB of the adrenals, TB of the liver, TB of the bones and joints (for example, TB spondylitis or Pott's Disease), TB lymphadenitis, and TB of the mouth, middle ear, larynx, and bronchial tree.
Conventional therapy for TB includes treatment with regimens containing pyrazinamide, isoniazid, ethambutol, streptomycin, rifampin, rifabutin, clarithromycin, ciprofloxacin, clofazamine, azithromycin, ethionamide, amikacin and resorcinomycin A. To treat latent (inactive) TB infection, isoniazid may be used alone. However, the usual initial treatment for pulmonary tuberculosis includes isoniazid in combination with at least one other drug, such as ethambutol, streptomycin, rifampin or ethionamide. Retreatment of pulmonary tuberculosis typically involves drug combinations including rifampin and other drugs as noted above. Development of resistance of the causative agent to anti-TB drugs, especially isoniazid, is well known. Extrapulmonary tuberculosis is also usually treated with a combination including rifampin and at least one of the other three drugs mentioned.
Mycobacterium Avium Complex (MAC)
M. avium and M. intracellulare are members of the Mycobacterium avium complex (MAC). M. paratuberculosis is a subspecies of M. avium and is also generally included in the MAC. These species have become increasingly important in recent years because of the high prevalence of disseminated MAC infection in AIDS patients. The Mycobacterium avium complex is comprised of 28 serovars which are distinguishable on the basis of their biochemical and seroagglutination characteristics (see review by Inderlied, et al. 1993. Clin. Microbial. Rev. 6, 266-310). Depending on the method of classification, 10-12 of the 28 serovars are classified as belonging to the species Mycobacterium avium, and 10-12 belong to the species Mycobacterium intracellulare. Six of the MAC serovars have not yet been definitively classified. MAC infections currently account for approximately 50% of the pathogenic isolates identified by mycobacteriology labs and are most common among AIDS and other immuno-compromised patients. Early diagnosis and treatment of MAC infections can improve and prolong the lives of infected individuals.
Anthrax and Anthrax Toxin
Anthrax toxin, produced by the gram positive rod-shaped aerobic, spore-forming bacterium Bacillus anthracis, is the toxic virulence factor secreted by this organism. B. anthraxis is often considered for use as a biological weapon due to the potency of the secreted exotoxin, and to the capacity of the bacterium to form dormant spores which resist harsh environmental conditions. Sporulation enables ready transport and distribution of large quantities of toxin-producing bacteria. The toxin is actually a composite consisting of 3 separate secreted proteins from the bacterium. The 3 proteins are protective antigen (PA), lethal factor (LF), and edema factor (EF). While LF and EF directly damage cells and cause disease, the PA is the focus of this disclosure. PA is crucial to the virulence of anthrax toxin, since the PA molecule is designed to import both LF and EF inside the membranes of cells. In the absence of PA-induced intracellular transport, anthrax toxin is unable to effect tissue destruction, since LF and EF only function from within the cell. The importance of PA in the function of anthrax toxin is underscored by the effective use of PA as the immunogen in anthrax vaccine. By generating an immune response against PA, the vaccine confers protection against full (3 component) anthrax toxin.
A closer examination of the interaction between PA and the host cells attacked by anthrax toxin is instructive. PA is first secreted as an 83 kDa monomeric polypeptide by B. anthracis in a large and functionally inactive form. This inactive PA binds to a mammalian receptor on the surface of host cells. The PA receptor has recently been isolated and sequenced, and found to possess von Willebrand Factor-like regions. After docking on the surface of host cells, PA interacts with a protease present on the cell surface. The protease processes the large and inactive PA molecule into a smaller and active 63 kDa fragment. The C-terminal 63 kDa fragment (PA63) remains bound to the cell and the N-terminal 20 kDa (PA20) dissociates from PA63. The identity of the protease has been the focus of scant research effort, and it is poorly characterized. However, prior studies have shown that the protease has characteristics that suggest it is a host-derived serine protease. A possible serine protease candidate noted in the literature is related to furine (itself a serine protease), but other serine proteases, such as elastase, proteinase-3, clostripain, or trypsin are possible alternatives (Molloy, S. S. et al. J Biol Chem 267, 16396-16402 (1992)). This proteolytic cleavage and subsequent dissociation of PA20 confer two new properties on PA63: (1) the ability to oligomerize into a ring-shaped heptameric SDS-dissociable structure termed prepore and (2) the ability to bind EF and LF. Oligomers containing PA63-EF, PA63-LF, or a combination of PA63-EF and PA63-LF are endocytosed and trafficked to an acidic compartment, where the PA63 prepore inserts into the membrane and forms a pore. During or after pore formation, EF and LF are translocated across the endosomal membrane into the cytoplasm. EF is a calmodulin-dependent adenylate cyclase which may protect the bacteria from destruction by phagocytes. LF is a metalloprotease that can kill macrophages or, at lower concentrations, induce macrophages to overproduce cytokines, possibly resulting in death of the host. These heptamers function as the transport vehicle to deliver LF and EF inside of the cell. Once inside the cell, LF and EF initiate abnormalities in cell function.
Because of some of the difficulties and inadequacies of conventional therapy for tuburculosis, other mycobacterial infections, and anthrax, new therapeutic modalities are desirable.
The inventor discloses a novel method of use for serine protease inhibitors as therapeutic agents to treat infections caused by tuberculosis (TB) and mycobacterium avium complex (MAC). These are intracellular human pathogens that establish infection and prolonged latency by infecting and surviving within human macrophages. Therefore, blocking the internalization of TB or MAC within macrophages is a novel approach to therapy vs these infectious agents. In an infectivity assay, the inventors have shown that α1-antitrypsin significantly inhibited both TB and MAC infection of human monocyte-derived-macrophages (MDM).
A novel approach to nullify the action of anthrax toxin is to block access of the toxin to the interior of the cell by interfering with the action of the host-derived serine protease that resides on the cell surface.
This invention thus addresses a long-felt need for safe and effective methods of treatment of tuburculosis, other mycobacterial infections, other Gram negative and Gram positive bacterial infections, and anthrax.